Skull conductivity and extension of head volume conductor model: simulation of bioelectric phenomena

Brain electrical activity is spatially distributed over three dimensions and actually spreads within the whole volume conductor. However, under certain circumstances, it is possible to limit the volume within which the study can be done. Given its high resistivity, the skull limits the spread of bioelectrical currents due to brain sources and it leaves only few holes for current flow. A simulation study performed adopting realistic head models is here presented. Test models were extended to different percentages of the reference volume conductor model (extended to the chin). Skull conductivity was set either 1/80 or 1/15 of brain conductivity. Scalp potential distributions were compared between each reduced model and the reference model by means of relative-difference measure (RDM). RDM obtained with the two skull/brain conductivity ratios were then compared to evaluate the role of skull electric property vs. skull morphology, for the definition of the required extension of the reduced model. The found differences increased non linearly with model volume reduction, dramatically augmenting as the lower skull was removed. The same model performed differently according to source position and orientation.